Two-material mold for the vertical hot-top continuous casting of metals

ABSTRACT

The mold includes a cooled copper body 4 on top of which sits a refractory feed head 9 which itself consists of two separate components--a thermally insulating upper bush 16 and a bottom annulus 17 made of a stronger and more compact refractory which is surrounded by a crimping ring 18 narrower than the annulus, so that an edge of the annulus extends beyond each side of the ring. As the lower inner edge of the bottom annulus becomes worn due to an erosive flow of molten metal, the working life of the annulus may be extended in two ways. First, the annulus may be removed between casting operations and its bottom surface may be faced. Second, after the facing operations have rendered the bottom surface flush with the crimping ring 18, the annulus may then be inverted so that the lower inner edge is replaced with the upper inner edge.

BACKGROUND OF THE INVENTION

The invention relates to the vertical hot-top continuous casting ofmetals, particularly steel.

DESCRIPTION OF THE PRIOR ART

It is known that vertical hot-top continuous casting is essentiallydistinguished from conventional vertical continuous casting by the factthat sitting on top of the mold body, made of copper or a copper alloy,vigorously cooled by the circulation of water and defining a passage forthe metal to be cast which undergoes peripheral solidification oncontact with its wall, is a feed head made of a thermally insulatingrefractory intended to contain, in the liquid state, a volume of castmetal delivered by the tundish placed above it (BF 2,000,365).

Thus, by virtue of a "two-material" mold of this type, it is possiblefor the free surface (meniscus) of the cast metal, which then lieswithin the feed head, to be distanced from the point where the castmetal on contact with the cold wall necessarily starts to solidify,namely the upper edge of the copper component.

Thus, the aim is to produce, by continuous casting, semifinishedproducts of a higher quality and with high extraction rates, which areeven higher than in conventional continuous casting. This is because anyhydrodynamic turbulence caused by the influx of molten metal into themold is confined within the refractory feed head so that, below it,solidification can commence and continue in a calm environment in whichthe cast steel progresses towards the output end of the mold without anysignificant velocity gradients in the section ("plug"-type flow).

It has been envisaged to design the refractory feed head itself in twosuperposed separate parts. An upper part--the bush--made of a refractorywhich is thermally very insulating, and therefore generally made of afibrous refractory which has quite a low density in order to prevent anyspurious solidification on the internal wall of the bush by the castmetal cooling when coming into contact with it, and a lower part, ofsmaller size and internally aligned with the mold body--theannulus--made of a compact refractory, and therefore having goodmechanical strength so as to withstand the mechanical erosion caused bythe proximity of the tip of incipient solidification on the upper edgeof the copper body in contact with the end of the feed head. An exampleof a compact refractory which may be suitable for this purpose is SiAlON(or Sialon®).

Other materials may also be suitable, but they all have the drawback ofbeing expensive to use since, although very strong, they end up by beingworn away, which means that the used annulus has to be replaced by a newannulus after a relatively short period of time.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a solution whichallows the working life of a refractory annulus of this type to be verysubstantially extended so as to make the running costs associated withrenewing this annulus in a hot-top continuous casting machinereasonable.

For this purpose, the subject of the invention is a two-material moldfor the vertical hot-top continuous casting of metals, such as steel,comprising a metal body (made of copper or copper alloy), which isvigorously cooled and defines a passage for the metal to be cast which,on contact with its internal wall, undergoes peripheral solidification,and a feed head made of a thermally insulating refractory which sits onthe cooled metal body and is intended to contain, in the liquid state,the molten metal poured from a tundish placed above it, which mold isdistinguished by the fact that the feed head is formed by two superposedseparate refractory parts--an upper bush made of a refractory havinggood thermal insulation properties and a lower annulus made of arefractory material having good mechanical strength properties andinternally aligned with the mold body--and that the annulus is crimpedinto a reinforcing means whose height is less than that of the annulus.

When casting long products, which will be the sole case consideredbelow, this reinforcing means advantageously consists of a crimpingring, preferably made of steel, surrounding the annulus.

As will have been understood, the invention therefore consists incrimping the hard refractory annulus (made of SiAlON)--which otherwisemight crack or even shatter after only a few casting runs--by means of areinforcing ring, for example made of steel, which surrounds only themiddle part of the perimeter of the annulus so as to leave a freeportion of the latter to extend above and below the reinforcing ring.

By virtue of such an arrangement, the lower edge of the annulus incontact with the cast metal may be readily restored by facing the faceturned toward the cooled metal body if degradation or spalling of theannulus is observed.

Furthermore, taking into account the potential symmetry of thearrangement, after the successive uses have exhausted the capacity ofthe lower face of the annulus to be regenerated, it is easy to turn thelatter upside down, thereby inverting the upper face and lower face, fora new series of casting runs. It is thus possible to double the workinglife of the compact-refractory annulus and therefore to halve its effecton the cost of running the casting machine.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be clearly understood and other aspects andadvantages will be more clearly apparent in light of the followingdescription given by way of an embodiment with reference to the appendedsingle plate of drawings, in which:

FIG. 1 shows diagrammatically, in vertical section, the top of avertical hot-top continuous casting machine for casting steel billets;

FIG. 2 shows, in partial vertical section, the detail of the upper partof a vertical hot-top continuous casting mold according to theinvention.

In the figures, the same components are denoted by identical referencenumbers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the general view in FIG. 1, it may be seen that the upperpart of a machine for the vertical hot-top continuous casting of steelconsists, in the direction of extraction of the metal to be produced,i.e. from the top downward in the figure, of a tundish 1 containing abath of molten metal 2 which it delivers to a mold 3 (or several molds3) placed beneath it by means of an outlet orifice (or several outletorifices) extended by a guide nozzle 20.

As may be seen, the mold comprises a tubular copper body 4 which isvigorously cooled by the circulation of water over the length of itsexternal face. Conventionally, a steel liner 5 is provided forchanneling this circulation and communicates at its ends with an inletchamber 6 and with a discharge chamber 7, these chambers being delimitedby a casing 8 surrounding the cooled metal body 4 at some distancetherefrom.

Sitting on top of the latter is a feed head 9 made of an uncooledrefractory, the internal wall of which is preferably aligned with thatof the body 4.

In the context of the casting process, the "cooled metal body 4 precededby the insulating refractory feed head 9" arrangement defines a passagefor the cast metal, the upper part of which passage, within the feedhead, is a region 12 for confining the hydrodynamic perturbations causedby the arrival of the stream 11 of molten metal into the mold and thelower part of which passage, which extends it, is a region 13 forsolidification of the cast metal. This solidification, as will be seen,begins right from the first contact of the cast steel with the internalwall of the cooled copper body 4, namely along the upper edge 14 of thiswall, and continues downstream, forming a solid shell 15 which grows inthickness from the periphery toward the centre. On leaving the mold, theshell 15, which has a thickness of slightly more than one centimeter, isstrong enough to withstand the ferrostatic pressure of the still-liquidcore and continues its centripetal growth until the cast semifinishedproduct 10 has completely solidified under the effect of the water sprayunits (not shown) which are located in the bottom half of the machine.Once the semifinished product has completely solidified, it is cut intoportions of the desired width (billets, blooms or slabs, depending onthe format of the cast section) and these portions are then availablefor subsequent forming operations (rolling, etc.).

Referring now more particularly to FIG. 2, it may be seen that therefractory feed head 9 is itself formed by stacking two separatecomponents:

an upper component--the bush 16--made of a refractory chosen for itsthermal insulation properties, since it has to prevent any prematurespurious solidification of the cast metal in the turbulence region 12.The material of choice will be an alumina-based fibrous refractory, forexample the material sold under the name A 120K by the French companyKAPYROK s.a.; and

a lower component--the annulus 17--made of a refractory chosen for itsgood mechanical strength since, in the vicinity of the crystallizer 4,it has to withstand as best as possible the mechanical erosion by theupper tip of the solid shell 15 on the edge 14 while the whole systemundergoes a vertical oscillatory motion which, as is known, is necessaryfor the success of the casting operation, and the thermomechanicalstresses of a machine operating in thermal cycles imposed by thenecessarily sequential nature of the casting process.

This annulus 17, for example made of SiAlON, preferably doped with boronnitride, as sold by the company VESUVIUS under the reference 531, iscrimped in a metal ring 18 in the factory by hot fitting. This avoidsthe risk of the annulus cracking or shattering, something which mayotherwise occur after a small number of casting runs, or even after eachcasting run. According to the invention, this crimping ring is placedaround the annulus and has a width dimension so as to allow the latterto extend on either side of the confines of the ring.

In order to be specific, it will be possible, for example, to surroundan annulus 17 three centimeters in height using a steel ring 18 two anda half centimeters wide placed around the perimeter of the annulus in acentral (and preferably symmetrical) position so as to allow the annulusto extend beyond the ring by a distance of two and a half millimeters oneach side.

As was mentioned, this arrangement makes it possible, in the event ofdamage or of spalling, to restore the face 19 of the annulus turnedtoward the metal body 4 by facing. Ordinarily, a facing operationconsumes a thickness of 0.1 to 0.2 mm of material.

Furthermore, once the limit of possible successive facing operations isreached, and the lower surface is therefore flush with the ring 18, itis easy to turn the annulus upside down and thus to return to the samesituation as the starting situation for a new series of casting runsbefore the worn annulus has to be changed for a new annulus.

It goes without saying that the invention is not limited to the exampledescribed above but extends to many variants or equivalents as long asthe essential characteristics of the invention, which are given in theappended claims, are respected.

In particular, "ring" in the sense of the present description should beunderstood to mean not only a continuous hoop, which may be put in placearound an annulus 17 of circular shape in the case of the verticalcontinuous casting of long products (blooms and billets), but also anyclamping means which provides the refractory annulus with mechanicalreinforcement allowing it to better withstand the high thermomechanicalstresses that it experiences because of the cyclic nature of the castingoperations.

In the case of the continuous casting of flat products or of productshaving a highly elongate crosssection, especially slabs, and employing arefractory annulus which is no longer continuous around the perimeter ofthe mold, but formed by a juxtaposition of segments, for example, thisreinforcing means may then advantageously consist of a U-clamp whichcompresses, by its ends, the abutting faces of each segment, the centralpart extending over the external face of the segment.

Moreover, although it is preferred, the arrangement shown in thefigures, which shows that there is alignment between the feed head andthe cooled metal body, is not essential for the implementation of theinvention.

What is claimed is:
 1. A two-material mold for the hot-top continuouscasting of metals, comprising: a metal body which is vigorously cooledand defines a passage for metal to be cast which, on contact with itsinternal wall, undergoes peripheral solidification, and a feed head madeof a thermally insulating refractory which sits on top of the metal bodyand is adapted to contain molten metal introduced into the mold, thefeed head being formed by two stacked internally-aligned and separablerefractory components, including an upper bush made of a refractoryhaving high thermal insulation properties, and a lower annulus made of arefractory having higher mechanical strength properties than said upperbush, the lower annulus having a reinforcing member surrounding itsexterior, said member having a height less than that of the annulus andsaid annulus having opposing end faces for engaging said upper bush andsaid metal body, respectively, and a symmetrical shape with respect to aplane extending between and parallel to said end faces such that eitherend face may engage said metal body.
 2. The mold as claimed in claim 1,wherein the reinforcing member is in a central position, centered withrespect to the height of the annulus.
 3. The mold as claimed in claim 1,wherein the reinforcing member is a crimping ring surrounding an annulusof circular shape.